The present invention concerns a programmable sequential-code recognition circuit for selecting a specific mode of a chip including a test mode in a semiconductor device having a plurality of operating modes.
With a semiconductor memory gradually tending to high integration and high reliability, the semiconductor chip has contained circuits for measuring various electrical properties of the chip, or various test modes, including a normal read/write mode. Such a special mode circuit is formed so as not to function in the normal read/write mode, thereby not affecting the inside of the chip. Usually, the special mode circuit serves as a buffer to connect a signal to the inside of the chip in the voltage exceeding an externally applied voltage, or includes a sensing circuit to produce a signal for operating the special mode circuit while to stop functioning of the normal red/write circuit. In a conventional semiconductor device having a test mode for evaluating the properties of the chip and other special modes together with the noraml read/write mode, the mode selection is accomplished by using a specially prepared pad or by adding a high voltage sensing circuit to an address/control pad.
Such a specially prepared pad is added to the pad used for the normal read/write mode, and is supplied with a power source to select the special mode. On the other hand, when using the high voltage sensing circuit, the high voltage (12-14V) is applied to the address/control pad connected with the high voltage sensing circuit, which then operates to select the special mode.
The problem arising from using the specially prepared pad is that each special mode needs each separate pad to thereby increase the chip size, and when packaging, the specially prepared pads are not often bonded, resulting in failure of test or increase of the number of the packaging pins. Furthermore, the method using the high voltage sensing circuit has a problem that it needs a separate high voltage source.